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Sample Chapter�

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�� John Michael Powell Mike’s Flight Deck Books Lafayette, CA 94549-0778

Sample Chapter�

Copyright ©2004 by John Michael Powell. All right reserved. Published by Mike’s Flight Deck Books™ www.mikesflightdeckbooks.com Printed in the United States of America. No part of this book, except the micro controller firmware, may be reproduced or transmitted in any form, or stored in any database or information retrieval system without the prior written permission of Mike’s Flight Deck Books™ or the author. Reviewers are, however, permitted to quote brief passages for inclusion in a review. Portions or all of the micro controller firmware may be stored in a computer system and may be incorporated in projects built or developed by the reader for non-commercial purposes.

The projects described in this book are intended for hobby purposes. Neither the designs nor the suggested construction methods are appropriate for applications in which failure could result in harm to people or damage to property.

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The information in this book, including firmware, circuit diagrams, product documentation or other information, is provided as is, without warranty of any kind, expressed or implied, including without limitation any warranty with regard to the accuracy, adequacy or completeness of material or the results obtained from using the material.

TurboCAD is a registered trademark of IMSI, Inc. Windows, Win32, and Visual C++ are registered trademarks of Microsoft Corporation in the United States and/or other countries. Adobe Acrobat is a registered trademark of Adobe Systems Incorporated. Teflon and Delrin are registered trademarks of E. I. du Pont de Nemours and Company. MuMetal is a registered trademark of Spang Specialty Metal Division of Spang & Company. Other terms used in this book which are believed to be trademarks are capitalized and may the property of their respective owners.

Sample Chapter�

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Preface ��

Table of Contents��

Introduction �� Air-Core Movements.................................................................................................... xi Servos........................................................................................................................... xii Stepping Motors........................................................................................................... xii Interfacing.................................................................................................................... xii Appendices ................................................................................................................. xiii Tools & Safety ............................................................................................................ xiii

Air-Core Movements �� How an Air-Core Movement Works............................................................................. 1

Operating Forces in Theory ....................................................................................... 2 Operating Forces in Practice...................................................................................... 4 Limiting Forces .......................................................................................................... 5

Driving an Air-Core Movement.................................................................................... 6 Commercial Driver Chips.......................................................................................... 7 Driving Without Chips.............................................................................................. 9 DIY Approaches....................................................................................................... 10

Sources of Air-Core Movements ................................................................................. 12

Air-Core Construction �� Overview...................................................................................................................... 13 Construction Technique.............................................................................................. 14

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Making the Rotor..................................................................................................... 15 Making the Body...................................................................................................... 20 Final Assembly......................................................................................................... 25

Air-Core Movement Bill of Materials ......................................................................... 28

Air-Core Airspeed Indicator ��"# Mechanical Overview.................................................................................................. 29 Mechanical Construction ............................................................................................ 31

Faceplate Assembly.................................................................................................. 31 Circuit Deck............................................................................................................. 36 Rear Deck................................................................................................................. 36

Overall Mechanical Assembly..................................................................................... 36 Dimensioned Deck Drawings....................................................................................... 37 Electronics.................................................................................................................... 40

Circuit Description .................................................................................................. 41 Circuit Construction................................................................................................ 46

Air-Core Airspeed Indicator Bill of Materials............................................................. 47

Air-Core Vertical Speed Indicator ��$# Mechanical Overview.................................................................................................. 49 Mechanical Construction ............................................................................................ 50

Faceplate Assembly.................................................................................................. 50 Faceplate graphics .................................................................................................... 51 Indicator................................................................................................................... 52 Circuit Deck............................................................................................................. 52 Rear Deck................................................................................................................. 52 Adjusting The Pointer ............................................................................................. 52

Dimensioned Deck Drawings....................................................................................... 53 Electronics.................................................................................................................... 56 Firmware ...................................................................................................................... 58

The Systems Level Perspective................................................................................ 58 The Firmware Framework........................................................................................ 59 Project Specific Firmware ........................................................................................ 61

Command Summary .................................................................................................... 63 Reset......................................................................................................................... 63 Set Display Value..................................................................................................... 63 Set Display Value (Frame Transfer) ........................................................................ 63 Query Name............................................................................................................. 64 Rename .................................................................................................................... 64 Set Frame Slot.......................................................................................................... 64

Firmware Listing .......................................................................................................... 64 Air-Core Vertical Speed Indicator Bill of Materials ................................................... 84

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Servo Principles ��%& RC Servos .................................................................................................................... 86 Servo Principles ........................................................................................................... 89

The Simple Proportional Loop ................................................................................ 89 Adding a Rate of Change Signal ............................................................................. 90 Adding an Accumulated Error Signal ..................................................................... 91 Adding a Dead Zone ................................................................................................ 91 Implementing Servo Loops ...................................................................................... 92

Motors .......................................................................................................................... 93 Permanent Magnet DC Motors ............................................................................... 93 Stepping Motors....................................................................................................... 94

Position Sensing........................................................................................................... 94 Potentiometers ......................................................................................................... 94

Shafts, Bearings & Bushings ........................................................................................ 96 Stepped profile shafts ............................................................................................... 96 Steel Shafts .............................................................................................................. 97 Supporting the Shaft................................................................................................ 98 Flexible Couplings ................................................................................................... 99

Gears ............................................................................................................................ 99 Key Points ................................................................................................................ 99 Gear Pitch and Module.......................................................................................... 100 Tooth Shape .......................................................................................................... 101 Materials ................................................................................................................ 101 Gears Systems ........................................................................................................ 101

Servo Turn Coordinator �� '� Mechanical Overview................................................................................................ 103 Mechanical Construction .......................................................................................... 105

Faceplate Assembly................................................................................................ 105 Servo Deck............................................................................................................. 105

Dimensioned Deck Drawings..................................................................................... 110 Electronics.................................................................................................................. 113

Pulse Generator ..................................................................................................... 113 Input Buffer............................................................................................................ 115 Power Conditioning............................................................................................... 116 Electronic Construction Technique ...................................................................... 116

Turn Coordinator Bill of Materials............................................................................ 117

Servo Attitude Indicator�� # Mechanical Overview................................................................................................ 119 Mechanical Construction .......................................................................................... 120


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Faceplate Assembly................................................................................................ 120 Yoke Assembly....................................................................................................... 124 Roll Bushing Deck ................................................................................................. 133 Circuit Deck........................................................................................................... 134 Roll Servo Deck..................................................................................................... 134

Dimensioned Deck Drawings..................................................................................... 136 Attitude Indicator Bill of Materials ........................................................................... 140

Servo VOR/GS CDI �� $� Mechanical Overview................................................................................................ 141 Mechanical Construction .......................................................................................... 142

Faceplate Assembly................................................................................................ 142 Circuit Decks ......................................................................................................... 144 Servo Deck............................................................................................................. 144


Rotary Encoders ..................................................................................................... 151 Multiplexed Seven-Segment Displays ................................................................... 152 Building the Electronics......................................................................................... 156

Firmware .................................................................................................................... 156 Overview................................................................................................................ 156 Interrupt Service Code .......................................................................................... 157 Setting Servo Movement Limits............................................................................ 160

Command Summary .................................................................................................. 161 Reset....................................................................................................................... 161 Set Servo Positions ................................................................................................ 161 Set Servo Positions (Frame Transfer).................................................................... 161 Set Digits................................................................................................................ 162 Set LEDs ................................................................................................................ 162 Query OBS............................................................................................................. 162 Query Name........................................................................................................... 162 Rename .................................................................................................................. 162 Set Frame Slot........................................................................................................ 162 Set Limits ............................................................................................................... 163

Firmware Listing ........................................................................................................ 163 Turn Coordinator Bill of Materials............................................................................ 185

Stepping Motors �� %( How Stepping Motors Step........................................................................................ 187

The Wave Step Sequence...................................................................................... 188 Two Phase Stepping............................................................................................... 189 Half Stepping ......................................................................................................... 190


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Micro Stepping ...................................................................................................... 191 Dynamic Effects: Starting, Accelerating and Running.......................................... 191

Stepping Motor Types ............................................................................................... 193 Can Stack Stepping Motors................................................................................... 193 Hybrid Stepping Motors ........................................................................................ 195 Winding Configurations ........................................................................................ 197

Driving Stepping Motors ........................................................................................... 199 Motor Voltage Selection ....................................................................................... 199 Discrete Unipolar Driver ....................................................................................... 199 Integrated Unipolar Driver .................................................................................... 200 Integrated Unipolar Driver with Logic.................................................................. 200 Discrete Stepping Logic ......................................................................................... 201 Discrete Bipolar Driver .......................................................................................... 202 Integrated Bipolar Driver....................................................................................... 203 Micro Stepping and Pulse Width Modulation ...................................................... 204

Sources ....................................................................................................................... 206 Of Motors............................................................................................................... 206 Of Information....................................................................................................... 206

Stepping Motor Gyro Compass �� "'( Mechanical Overview................................................................................................ 208 Mechanical Construction .......................................................................................... 209

Faceplate Assembly................................................................................................ 209 Compass Rose ........................................................................................................ 210 Interrupter Flag ...................................................................................................... 211 Motor Deck............................................................................................................ 212

Dimensioned Drawings .............................................................................................. 214 Electronics.................................................................................................................. 217 Firmware .................................................................................................................... 220

Overview................................................................................................................ 220 Compass Card Reset .............................................................................................. 220 Rotary Encoder Support......................................................................................... 221 Motor Movement................................................................................................... 222

Command Summary .................................................................................................. 223 Reset....................................................................................................................... 223 Set Motor Position................................................................................................. 223 Set Motor Position (frame transfer)....................................................................... 224 Query Name........................................................................................................... 224 Rename .................................................................................................................. 224 Set Frame Slot........................................................................................................ 224

Firmware Listing ........................................................................................................ 224 Stepping Motor Gyro Compass Bill of Materials....................................................... 242


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Stepping Motor ADF/RMI Head ��"$� Mechanical Overview................................................................................................ 244

Gear Assembly Operation ..................................................................................... 244 Mechanical Construction .......................................................................................... 245

Gear Shafts............................................................................................................. 245 Assembling the Gear Train ................................................................................... 249 Installing the Optical Interrupters......................................................................... 250 Faceplate Assembly................................................................................................ 250

Dimensioned Deck Drawings..................................................................................... 253 Three Inch ADF Version....................................................................................... 254 Two Inch Dual Pointer Version Drawings ............................................................ 259

Electronics.................................................................................................................. 263 Firmware .................................................................................................................... 265 Command Summary .................................................................................................. 266

Reset....................................................................................................................... 266 Set Indicator Positions........................................................................................... 266 Set Motor Positions (frame transfer) ..................................................................... 266 Query Offset........................................................................................................... 267 Query Name........................................................................................................... 267 Rename .................................................................................................................. 267 Set Limits ............................................................................................................... 267 Set Frame Slot........................................................................................................ 267

Firmware Listing ........................................................................................................ 268 Stepping Motor ADF/RMI Head Bill of Materials .................................................... 293

Stepping Motor Sensitive Altimeter ��"#& Mechanical Overview................................................................................................ 295


Faceplate Assembly................................................................................................ 297 Gear Shafts............................................................................................................. 298 Constructing the Gear Assembly........................................................................... 301 Coupling to the Motor........................................................................................... 306 Pointers .................................................................................................................. 307

Dimensioned Deck Drawings..................................................................................... 310 Electronics.................................................................................................................. 314 Command Summary .................................................................................................. 316

Reset....................................................................................................................... 316 Set Motor Position................................................................................................. 316 Set Motor Position (Frame Transfer) .................................................................... 316 Query Name........................................................................................................... 316 Rename .................................................................................................................. 317 Set Frame Slot........................................................................................................ 317


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Firmware .................................................................................................................... 317 Firmware Listing ........................................................................................................ 320 Stepping Motor Altimeter Bill of Materials .............................................................. 335

Stepping Motor Turbine Tachometer �� ( Mechanical Overview................................................................................................ 338


Faceplate Assembly................................................................................................ 340 Gear Shafts............................................................................................................. 341 Pointers .................................................................................................................. 343 Final Mechanical Assembly................................................................................... 343


Detailed Circuit Operation.................................................................................... 352 Building the Electronics......................................................................................... 354 Changing the Input Range .................................................................................... 356 Calibration............................................................................................................. 357

Stepping Motor Turbine Tachometer Bill of Materials............................................. 358

Interfacing �� )� Really Useful Background Skills ................................................................................ 361

C/C++ Programming ............................................................................................. 361 Win32 API............................................................................................................. 362

Getting Data Out of Your Simulator ......................................................................... 363 Falcon 4.0............................................................................................................... 363 Lock-On: Modern Air Combat.............................................................................. 363 Microsoft Flight Simulator..................................................................................... 363 Rowan’s Battle of Britain....................................................................................... 364 Sailors of the Sky ................................................................................................... 364 X-Plane .................................................................................................................. 364

The Serial Com Port Under Win32 .......................................................................... 365 Update Frequency ...................................................................................................... 365 Sharing a Serial Com Port ......................................................................................... 366

Port Expander ........................................................................................................ 366 Port Expander Bill of Materials.................................................................................. 372

Appendix 1: Precision Layout �� (� Step 1: NOT Measuring with a Yardstick.................................................................. 373 Step 2: NOT Marking with Chalk. ........................................................................... 374 Step 3: NOT Cutting with an Axe............................................................................ 375


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Drilling Templates ..................................................................................................... 376

Appendix 2: Standard Instrument Panel Cutouts �� (( One Inch Instruments................................................................................................ 377 Two Inch Instruments ............................................................................................... 379 Three Inch Instruments ............................................................................................. 380

Appendix 3: Information Resources �� %� Introductory Electronics and Construction Techniques ........................................... 381 Micro Controllers ...................................................................................................... 381 Windows Programming.............................................................................................. 382 Gears .......................................................................................................................... 383 Stepping Motors......................................................................................................... 383

Appendix 4: Material Sources ��%& Ball Bearings .............................................................................................................. 385 Brass Tubing............................................................................................................... 386 Electronic Parts .......................................................................................................... 386 Gears .......................................................................................................................... 387 Magnets...................................................................................................................... 388 Magnet Wire.............................................................................................................. 389 Motors, Stepping & DC............................................................................................. 390 Plastic......................................................................................................................... 391 RC Servos .................................................................................................................. 391 Sheet Aluminum ....................................................................................................... 392 Flight Rated Aircraft Instruments ............................................................................. 393

Index��#&

Colophon ��##

Ordering Information��$''

Sample Chapter�

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This project makes use of two strengths of using a stepping motor in a simulated instrument or gauge. There is substantial torque to work with and the motor can continually rotate without hitting a mechanical stop. This is a good match for the relatively large inertia of the compass rose disk, and the need to let it turn freely if you decide to fly in circles.

Of course this approach will work just as well with a low inertia pointer should you want to make an air speed indicator instead.

In addition to demonstrating the stepping motor in a simulated instrument, this project introduces the optical interrupter. It is used here to sense the zero position of the compass card during power up initialization and execution of the reset command. These will be valuable additions to your instrument simulation toolbox.


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��,� �� -�The side view in figure 1 shows the relative positions of the major components. The stepping motor is centered immediately behind the faceplate assembly. The compass rose mounts directly on the motor shaft. The rotary encoder mounts in a lower corner of the motor deck and is positioned such that its extended shaft comes through the front bezel where the lower left instrument mounting screw would otherwise be. Immediately above the motor is an optical interrupter. It mounts on the front surface of the deck with its leads pointed toward the circuit deck. The circuit deck is just behind the motor deck and is followed in turn by the rear deck.

Figure 1. A side view of the simulated directional gyroscope.

The stepping motor has no unique starting position. The motor shaft is in an unknown rotational position when powered up. The flag fastened to the compass rose support

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provides the means for the firmware to rotate the compass rose to a known position upon initialization.

��The project’s mechanical construction is reasonably straightforward. The only metal bending is forming the interrupter flag. Most of the effort goes into cutting flat metal pieces and drilling holes. Tolerances are not stringent. Nonetheless, there are some mounting holes that should line up during final assembly. Good layout technique and the use of a drilling template are recommended to help make this come about. Details of these suggestions are covered in the appendix on precision layout.

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The front bezel, lens, light diffuser and faceplate make up the faceplate assembly. It incorporates internal lighting based on either LEDs or incandescent lamps. Lighting options are covered in the air-core airspeed project chapter, and you are referred there for those particular details.

This assembly has only a few unique aspects relative to the other projects. The most visible difference is the white heading reference lines painted on the inside of the lens. This can be as simple as a single vertical white line bisecting the lens. Alternatively it can be a complex silhouette of an aircraft with the nose pointing to the heading on the compass rose.

The lens markings can be added using a permanent ink marking pen. A white ink pen such as the Sakura Pen Touch™ or equivalent works well. These pens are generally available from arts and crafts stores.

A template is very useful. Transparency material used for overhead projectors is a good material for the template. The edges of the holes act as pen guides. Placing several layers of masking tape on the lower side of the template next to the cuts will hold the template above the lens and help prevent the ink from wicking between the template and lens.


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Figure 2. Sample graphic for lens.

Unlike those in other projects, this faceplate deck has no markings on it. It simply has a hole large enough to clear the compass rose support. It’s the compass rose that carries the markings.

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The compass rose is an aluminum disk 2.7” in diameter. It’s supported on the motor shaft by a short length of ½” diameter aluminum rod. The support is glued with epoxy to the center of the disk. The easiest way to glue these parts together is to assemble the rest of the instrument first. It can then be used to hold the disk and support in proper alignment while the epoxy cures. This approach has the further advantage that it will compensate for small dimensional errors made while fabricating these two parts.

The compass rose graphic is, at least in basic form, simply white radial markings and lettering on a black background. You’ll get the nicest results if you generate it in a drawing program and print out an original. If you don’t have access to a drawing program you can make a copy of the following figure.


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Figure 3. Compass rose graphic.

The graphic can be mounted on the disk with double sided tape. Alternatively, if you plan on printing an original, you might consider printing on self adhesive paper. This is available from many office supply stores. It’s the same material labels are made from. It’s simply not scored into individual labels.

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The interrupter flag is an L-shaped piece of thin sheet metal that wraps around the rod supporting the compass rose. The flag is held in place by a machine screw and nut. The outer end of the flag is bent such that as the compass rose rotates, the flag passes through the optical interrupter gap. The flag is nominally made of 0.020” aluminum, but can be any sufficiently rigid opaque material. A strip cut from a discarded food can would work quite well, and has a decided price advantage over purchased material.


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Figure 4. Compass rose and interrupter flag assembly.

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The motor deck is the support for the rotary encoder, the optical interrupter, and the stepping motor. The holes for mounting the motor are positioned for a standard size motor that was commonly used in older floppy and some hard drives. The motor is 1.7 inches in square cross section and roughly ¾ of an inch long. This size turns up on the surplus market frequently as well. If you are uncertain that the motor you plan on using is this size, you might want to measure the location of the mounting holes on your motor before you fabricate the motor deck.

The motor mounts on the rear surface of the deck with the motor shaft pointing forward. There is no particular up or down orientation of the motor. Choose an orientation that positions the motor leads to your preference.


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The rotary encoder mounts in a ¼” hole in the lower left corner of the deck. The encoder body has an indexing pin for aligning the encoder and preventing it from turning after it is installed. After drilling the mounting hole in the deck, you will need to cut a small notch as shown below to accept the index pin. The notch should be made 45 degrees from vertical to position the encoder body so that it will clear the nearby threaded spacer.

The shaft of the rotary encoder must be extended to reach through the front bezel assembly. This is done with a split coupling made of a short length of 9/64” ID brass hobby tubing. Make a lengthwise cut through the wall of the tubing so that it has a “C” cross section. Squeeze the tubing so it slides tightly over the encoder shaft. A short length of 1/8” diameter hobby brass rod can be pushed into the other end of the tubing to extend the shaft.

Figure 5. Rotary encoder mounting hole and shaft extension.

The optical interrupter mounts on the front surface of the motor deck just above the motor. The leads go through holes in the deck. The interrupter has two mounting ears, but only the top is used. Use a ½” 4-40 machine screw and nut.


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Figure 6. Front bezel and lens.


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Figure 7. Light diffuser and faceplate deck.


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Figure 8. Motor deck and circuit deck.


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Figure 9. Rear deck and compass rose disk.

1� ��The circuitry is very similar to that of the VSI project. The motor used here has bipolar windings and can be driven using the same H-bridge chip. The same types of micro controller (PIC16F628) and RS232 transceiver (MAX232) are used. The only differences are the additions of a rotary encoder and an optical interrupter. Because real


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directional gyros drift, they have an adjustment knob. The rotary encoder serves that function on this simulated DG. Rather than making a mechanical adjustment however, the rotary encoder interfaces to the micro controller. The micro controller, in turn, directs the stepping motor to make the adjustment.

Stepping motors power up at random rotational positions. They have no inherent means of moving to a known “zero” position. An optical interrupter adds that capability. An optical interrupter is a combination of an LED and a phototransistor. With nothing blocking the light path, the light from the LED biases the phototransistor on. If something blocks the path, the phototransistor turns off. In this project, an arm clamped on the motor shaft moves a flag that can block the path. This allows firmware to sense when the shaft has rotated to its “zero” position.

When a real DG spins up it displays a random heading, so it may seem that a simulated unit would not have the ability to self-initialize. Quite likely, the random nature of the DG is handled within the simulation software on the host computer. The host computer then commands the simulated instrument to display a particular heading. If the simulated instrument cannot self-initialize, it will be out of sync with the software.

Furthermore, the self-initialing capability allows this instrument simulation approach to be used for a variety of instrument and gauge types. By replacing the compass rose disk with a pointer, this approach can provide you with a VSI, a tachometer, airspeed indicator or just about any single pointer type unit.

The voltage regulator that supplies power to the motor you use may be need to be different than the 7805 shown in the schematic. Depending on the actual motor you use, you may need to select a regulator with a different voltage rating. You don’t have to supply the full voltage the motor is rated for as it’s not necessary for the motor to develop full rated torque. It’s only spinning an aluminum disk. Use the lowest voltage that gives reliable operation. This will minimize current demands from your power supply.

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This is a fairly benign circuit as far as construction demands go. Follow a few basic guidelines and it should work fine. Printed circuit boards look very nice but are not a necessity. Perf board works fine. Generally, try to position parts to minimize the wiring lengths. Give precedence to parts that generate heat like voltage regulators and driver chips. Place those on the top edge of the board so the waste heat doesn’t heat the other components. Run the power and ground connections first. The power bypass capacitors work best when placed close to the power pins of the chips they bypass.

The one area that is most likely to cause you grief is the voltage regulator for the motor. Depending on your motor selection, the regulator may heat up. Heat sink this regulator. Bolting it to the rear deck is one possibility.


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If building electronics is new to you, there are a few references in the information resources appendix that may be of interest.

Figure 10. Stepping motor based directional gyroscope schematic.


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The firmware’s basic functionality is to put a stepping motor under the control of a host computer, to communicate with the host through a standard com port, and to provide a manually adjustable offset to the host commanded motor position. Additionally, it provides the ability to rotate the motor to a known “zero” position during initial power up or when the host issues a reset command.

The firmware is based upon the same framework covered in detail in the VSI chapter. The PIC spends most of its time in a tight polling loop checking flags used to manage communication with the host. A command from the host to rotate the compass disk does not directly move the stepping motor. Instead, the command loads a two byte variable representing the desired motor position. Motor movement is handled by the interrupt service code.

Very little is accomplished by a single pass through the interrupt service code. When initially powered up, each pass will result in a motor half-step toward the compass card zero position. During normal operation, each pass checks for changes in the rotary encoder state, and will move the motor a half-step if a compass card movement command is in process. Large effects accrue from many passes through this code.

One of the PIC’s internal timers generates a repeating interrupt that causes the periodic execution of the interrupt service code. It is the interval between interrupts that establishes the stepping speed of the motor and the response time of the rotary encoder.

What is actually a fairly simple approach is somewhat obscured by the volume of assembly code that implements it. The motor shaft can be rotated to any of 800 possible locations, requiring two bytes each to track the current and desired positions. The need to do 16 bit arithmetic on an eight bit micro controller further obscures the approach. Just keep in mind that the underlying approach is a simple one, and don’t loose the forest for the trees.

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When the PIC is first powered up it executes initialization code that, among other things, sets the flag “fSeekZero”. When the interrupt service code is run, fSeekZero indicates that the first order of business is to rotate the compass card to its zero position. Until this is done, the code for normal operation is skipped.

The zero position of the compass card is sensed by an optical interrupter. The firmware will half-step the motor until the interrupter flag just enters the gap. If the interrupter is already in the gap, the motor is first stepped the other direction until the interrupter is


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moved out of the gap. If this were not done, positioning could be off by up to the width of the interrupter flag.

Once the compass card has been initially positioned at zero, fSeekZero is cleared, opening the path to normal operation.

This flowchart illustrates the overall process rather than details of the assembly language.

Figure 11. Compass card reset flowchart.

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The rotary encoder, along with the bulk of the firmware used to read, debounce and decode it, is identical to what is used (and covered in great detail) in the VOR/GS CDI chapter.

Basically, each pass through the interrupt service code checks the encoder outputs for a change. When a change is detected, several more passes through the code are allowed to go by before taking a final reading on the encoder’s outputs. This allows the outputs to fully settle and avoids taking action on what might be electrical noise or mechanically bouncing switch contacts.

The encoder change is decoded by combining the new output state with the just previous output state to create a four bit “state transition vector”. This vector can reflect all possible changes in the encoder’s outputs including those that should not normally occur. The vector is added to the PIC’s program counter, effectively resulting in a 16-way branch. The sixteen endpoints of this branch direct firmware execution to the appropriate code. Essentially the decoding function is “hardwired” into the branch structure.


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The firmware times the interval since the last encoder change. The interval will be short if the encoder shaft is being turned rapidly. Rapid turning is indicative of the user’s desire to quickly change the orientation of the compass card. To make the adjustment function more responsive, rapidly turning the encoder results in disproportionately larger step changes to the compass card’s position. Slowly tuning the encoder results in small steps for accurate positioning.

The firmware uses two variables to track both the current and desired positions of the compass card. When the rotary encoder is used to move the compass card, the contents of POSITION, the variable tracking the current position, is increased or decreased. The firmware that actually manages motor movement will note the change and shift the compass card position.

Because motor movement commands from the host write to NEWPOS, the variable holding the desired position, a host sent movement command does not over-write or negate the effects of manual compass card adjustments. Only a reset command will do so.

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This project makes use of a 400 step per revolution stepping motor. The motor is half stepped to boost the number of steps to 800. The motor has bipolar windings and so is driven with an H-bridge. The micro controller supplies the four digital signals necessary to control the H-bridge.

During each pass through the interrupt service routine, the difference between the desired motor position and the current position is calculated. A difference of zero indicates no need for movement and the code executes a return from interrupt.

A non-zero difference is further examined to determine the best direction of movement. This is not simply a case of looking at the sign of the difference. The goal is to move in the direction that minimizes the required number of steps. A movement from 0 to 1 will go in one direction while a movement from 0 to 799 should clearly go the other way even though both are positive differences.

A tentative direction is set by looking at the sign of the difference, with a positive difference implying forward movement. The magnitude of the difference is then compared to the number of half-steps equivalent to half a rotation. If the magnitude is larger, it’s shorter to go the other way, and the direction flag is complemented.

With the direction determined, the current motor step variable, MOTORSTEP, is incremented or decremented as appropriate. The lower three bits are extracted and used as an index into a table containing the values needed to generate the next step. This value is written to PORTB, which is connected to the motor H-bridge power driver.


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Figure 12. How the motor is stepped.

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Bytes [0:4] Byte 5 ID 0 Reset forces the firmware to jump to the power on reset code. It will initialize all variables and the timers, and enable interrupts. The stepping motor will rotate to its zero position.

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Bytes [0:4] Byte 5 Bytes [6:7] ID 1 Motor position Set motor position loads the internal variable commanding the relative motor position. The physical position the motor will rotate to is the sum of the relative motor position


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and the offset set by turning the rotary encoder. After a reset command or a power on reset, the offset is zero. Motor position is a two-byte integer ranging from 0 to 799. Byte 6 is the low byte.

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Bytes [0:4] Byte 5 N Bytes Bytes [N+6:N+7] ID 2 X Motor position Set motor position (frame transfer) has the same effect as the set motor position command. The difference is in the data transfer. A frame transfer sends a vector of data to a group of instruments sharing the same broadcast ID. This is a more efficient use of I/O bandwidth as only a single ID-Command header is used for the group. Each instrument picks its data out of the vector from a unique slot. See command set frame slot.

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Bytes [0:4] Byte 5 ID 8 Query name causes the instrument to transmit the 17 bytes from the nonvolatile EEPROM memory containing the instrument’s unique ID and the firmware version.

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Bytes [0:4] Byte 5 Bytes [6:10] ID 9 New unique ID Rename writes the five-byte unique ID field stored in nonvolatile EEPROM. It also places a copy in the working registers so the instrument will immediately respond to the new ID.

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Bytes [0:4] Byte 5 Byte 6 ID 10 Slot Set frame slot writes the value used to index the instrument’s data position in the frame data transfer. This value is stored in nonvolatile EEPROM memory and the index stored in working register is initialized on power-on reset and on the reset command.

.��-�� 8��title "2 Phase driver for stepping motor based gauge/instrument" ; ; This application generates 4 drive signals for an L293D ; power driver which in turn provides bipolar drive ; for both phases of a bipolar stepping motor ; RB7 - Phase A high ; RB6 - Phase A low


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; RB5 - Phase B high ; RB4 - Phase B low ; RB3 - NC ; RB2 - RS232C data out ; RB1 - RS232C data in ; RB0 - NC ; RA3:7 - NC ; RA1:2 - Rotary encoder inputs ; RA0 - Optical interrupter input ; ; Hardware Notes: ; This application runs on a 16F628 executing at 12 MHz ; _MCLR is tied through a 10K Resistor to Vcc ; ; Mike Powell February 25, 2004 ; LIST R=DEC ifdef __16F628 INCLUDE "p16f628.inc" endif ; Registers CBLOCK 0x020 Holder0 ; Placeholder for 8 bytes that are used Holder1 ; in other projects. Having placeholders Holder2 ; keeps the READ NAME data format the same Holder3 ; across all the projects. Makes it easier Holder4 ; for lazy programmers like me to write Holder5 ; host interface code for these projects Holder6 ; Holder7 ; SkipCt ; # bytes to skip before picking data Name2Byte0 Name2Byte1 ; Unique instrument ID is stored Name2Byte2 ; here Name2Byte3 Name2Byte4 Name2Byte5 ENDC CBLOCK 0x030 NameBasePtr ; Points to start of ID1 field NBoffset ; Offset to name element with ID1 field NameByte0 NameByte1 ; Shared Instrument ID is stored here NameByte2 NameByte3 NameByte4 NameByte5 Name2BasePtr ; Points to start of ID2 field NB2offset ; Offset to name element with ID2 field


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fCMD ; flag indicating command being processed fDATA ; flag indicating serial comm moving data RXBUF ; Received data buffer DAToffset Count ; Used to count bytes transferred ENDC CBLOCK 0x040 BasePtr ; points to start of data buffer Data0 ; Buffer area used to collect command Data1 ; data before executing command Data2 Data3 Data4 Data5 Data6 Data7 Data8 Data9 DataA DataB DataCtr ; byte counter for data xfers ENDC CBLOCK 0x050 Pos:2 ; Tracks current position of motor NewPos:2 ; New position motor is to move toward Delta:2 ; Stores diff between current and new positions HalfWay:2 ; Constant # steps = 1 half motor revolution MaxPos:2 ; Constant # steps = 1 full motor revolution -1 RegPtr ; Pointer into register (data) memory EEPtr ; Pointer into EEPROM memory fQuery ; Flag indicating host query in progress QueryCtr ; Counter used in servicing host query SkipCtr ; number of data bytes skipped in frame xfer Temp ; Who knows? it's a temp value! FMoveBack ; flag indicating desired motor step direction fSeekzero ; flag to force motor position reset fDebounce ; flag indicating RE contact debounce in process DebounceCt ; counter tracking debounce process progress MotorStep ; index into the motor step pattern REState ; state variables used in decoding rotary encoder REStateTemp ; state changes RETime ; ENDC ifdef __16F628 __CONFIG _CP_OFF & _WDT_OFF & _XT_OSC & _PWRTE_ON & _LVP_OFF else __CONFIG _CP_OFF & _DEBUG_OFF & _XT_OSC & _PWRTE_ON & _WDT_OFF &


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_LVP_OFF & _WRT_ENABLE_ON & _BODEN_ON & _CPD_OFF endif PAGE ; Mainline of GYRO COMPASS SIM org 0 nop goto INITIALIZE org 4 ; ****INTERRUPT ENTRY POINT****** bcf PIR1, TMR2IF ; clear the timer2 interrupt flag movf fSeekzero, w ; btfsc STATUS, Z ; position reset in progress? goto OPERATE ; No: proceed with normal operations btfss fSeekzero, 1 ; in first portion of position reset? goto FINDDARK ; No: proceed to second portion btfsc PORTA, 0 ; Yes: are position sensors dark? 1 = dark goto INCRSTEP ; Yes: move forward a step bcf fSeekzero, 1 ; No: done with first portion of reset retfie FINDDARK btfss PORTA, 0 ; are position sensors dark? 1 = dark goto DECRSTEP ; No: back up another step clrf Pos ; Yes: clear POS and flag, ready to go now clrf Pos + 1 clrf fSeekzero ; done with position reset OPERATE ; This segment will look at the rotary encoder bits ; and look for a change. if not a counter will be decremented ; to time the interval between state changes. A slow ; rate will result in a fine adjustment movement. A Fast ; rate will result in a larger adjustment step. A detected ; change in state will start a debounce interval before ; sampling the new state. Once debounced the current and new ; state will be combined to interpret move forward, move ; back or illegal state transition and incr or decr the offset btfsc fDebounce, 0 goto DEBOUNCE movf PORTA, w ; read the two bits of the rotary encoder andlw 0x06 ; mask out anything else on the port subwf REState, w ; compare it to the last recorded state btfss STATUS, Z ; look at zero flag for equality comparison goto RECHANGE decfsz RETime, f ; count down RETime to see if slow rotations


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goto MOVEMENT bsf fDebounce, 1 ; no change for RETime, so set slow flag goto MOVEMENT RECHANGE bsf fDebounce, 0 ; There's a change so initiate contact debounce movlw 0x03 movwf DebounceCt ; Initialize debounce counter movlw 15 movwf RETime ; Initialize "slow" timer goto MOVEMENT DEBOUNCE decfsz DebounceCt, f goto MOVEMENT ; Debounce counter not zero yet rrf PORTA, w ; Read the rotary encoder bits, shift down andlw 0x03 ; and mask everything else out movwf REStateTemp ; and stuff into a temp register rlf REState, w andlw 0x0c addwf REStateTemp, w ; combine stored and new rotary encoder states andlw 0x0f ; make sure no other bits are on movwf Temp movlw HIGH RETABLE movwf PCLATH movf Temp, w addwf PCL, f RETABLE goto NOCHANGE ; State <00><00> goto ADJMINUS ; State <00><01> goto ADJPLUS ; State <00><10> goto NOCHANGE ; State <00><11> illegal transition goto ADJPLUS ; State <01><00> goto NOCHANGE ; State <01><01> goto NOCHANGE ; State <01><10> illegal transition goto ADJMINUS ; State <01><11> goto ADJMINUS ; State <10><00> goto NOCHANGE ; State <10><01> illegal transition goto NOCHANGE ; State <10><10> goto ADJPLUS ; State <10><11> goto NOCHANGE ; State <11><00> illegal transition goto ADJPLUS ; State <11><01> goto ADJMINUS ; State <11><10> goto NOCHANGE ; State <11><11> NOCHANGE bcf fDebounce, 0 ; clear the debounce flag movf PORTA, w ; Read and load current rotary encoder state andlw 0x06 movwf REState goto MOVEMENT


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ADJPLUS movf PORTA, w ; Read and load current rotary encoder state andlw 0x06 movwf REState btfss fDebounce, 1 ; is this a fast movement? goto FASTPLUS movlw 1 goto PLUS FASTPLUS movlw 8 PLUS clrf fDebounce addwf Pos, f btfss STATUS, C ; 16 bit arithmetic here goto CHECKPADJ movlw 1 addwf Pos + 1, f CHECKPADJ movf MaxPos, w ; check if this change has pushed Pos subwf Pos, w ; beyond the maximum. if so replace Pos movwf Delta ; with Pos - MaxPos (both bytes) btfss STATUS, C goto BORROW3 movf MaxPos + 1, w subwf Pos + 1, w movwf Delta + 1 btfsc STATUS, C goto TOOBIG3 ; Result postive, so Pos is too big goto MOVEMENT BORROW3 movlw 1 addwf MaxPos + 1, w subwf Pos + 1, w movwf Delta + 1 btfsc STATUS, C goto TOOBIG3 goto MOVEMENT TOOBIG3 ; replace pos with pos - maxpos movf Delta, w movwf Pos movf Delta + 1, w Movwf Pos+ 1 goto MOVEMENT ADJMINUS movf PORTA, w ; Read and load current rotary encoder state andlw 0x06 movwf REState btfss fDebounce, 1 ; is this a fast movement? goto FASTMINUS


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movlw 1 goto MINUS FASTMINUS movlw 8 MINUS clrf fDebounce subwf Pos, f btfsc STATUS, C ; 16 bit arithmetic here goto MOVEMENT movlw 1 subwf Pos + 1, f btfsc STATUS, C goto MOVEMENT movf MaxPos + 1, w ; Adjustment unflowed, add MaxPos to bring addwf Pos + 1, f ; Pos back into range movf MaxPos, w addwf Pos, f btfss STATUS, C goto ADDONE movlw 1 addwf Pos + 1 ADDONE movlw 1 addwf Pos, f btfss STATUS, C goto MOVEMENT addwf Pos + 1, f goto MOVEMENT MOVEMENT ; This segment determines the absolute difference between ; current and new positions this is 16 bit arithmetic ; being performed on an 8 bit machine bcf fMoveBack, 0 movf Pos, w ; first check the low bytes subwf NewPos, w movwf Delta btfss STATUS, C ; Carry set means positive result goto BORROW1 movf Pos + 1, w subwf NewPos + 1, w movwf Delta + 1 btfss STATUS, C goto ITSNEG goto ITSPOS BORROW1 movlw 1 addwf Pos + 1, w subwf NewPos + 1, w movwf Delta + 1


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btfsc STATUS, C goto ITSPOS ITSNEG bsf fMoveBack, 0 ; if the result is negative, complement it comf Delta + 1, f ; we want the absolute difference of movement comf Delta, f movlw 1 addwf Delta, f btfss STATUS, C goto ITSPOS movlw 1 addwf Delta + 1, f ITSPOS ; the result could be zero, if so we're done movf Delta, f btfss STATUS, Z goto CHECKSIZE movf Delta + 1, f btfsc STATUS, Z retfie CHECKSIZE movf Delta, w subwf HalfWay, w btfss STATUS, C ; CARRY set means positive result goto BORROW2 movf Delta + 1, w subwf HalfWay + 1, w btfss STATUS, C goto TOOBIG goto SIZEOK BORROW2 movlw 1 addwf Delta + 1, w subwf HalfWay + 1, w btfss STATUS, C goto TOOBIG goto SIZEOK TOOBIG movlw 1 xorwf fMoveBack, f SIZEOK btfss fMoveBack, 0 goto FORWARD BACKWARD movf Pos, f ; before moving backward see if position is at zero btfss STATUS, Z ; if so, set position to maximum position goto DECRPOS ; otherwise decrement position and take a step movf Pos + 1, f


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btfss STATUS, Z goto DECRPOS movf MaxPos, w movwf Pos movf MaxPos +1, w movwf Pos + 1 goto DECRSTEP DECRPOS movlw 1 subwf Pos, f btfsc STATUS, C goto DECRSTEP subwf Pos + 1, f DECRSTEP decf MotorStep, f goto TAKESTEP FORWARD movf MaxPos, w ; see if motor is already at maximum position subwf Pos, w ; if so, next postion is zero btfss STATUS, Z ; otherwise increment position and take a step goto INCRPOS movf MaxPos + 1, w subwf Pos + 1, w btfss STATUS, Z goto INCRPOS clrf Pos clrf Pos + 1 goto INCRSTEP INCRPOS movlw 1 addwf Pos, f btfss STATUS, C goto INCRSTEP addwf Pos + 1, f INCRSTEP incf MotorStep, f TAKESTEP movf MotorStep, w andlw 0x07 ; Look only at the lower 3 bits call NEXTSTEP ; get the step pattern for the drivers movwf PORTB ; and stuff it in the output retfie NEXTSTEP movwf Temp movlw HIGH STEPTABLE


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movwf PCLATH movf Temp, w addwf PCL, f ; add the table offset STEPTABLE retlw 0x090 ; step pattern for step 1 retlw 0x010 ; step pattern for step 2 retlw 0x050 ; step pattern for step 3 retlw 0x040 ; step pattern for step 4 retlw 0x060 ; step pattern for step 5 retlw 0x020 ; step pattern for step 6 retlw 0x0A0 ; step pattern for step 7 retlw 0x080 ; step pattern for step 0 ; ******************END OF INTERRUPT SERVICE************* INITIALIZE clrf Pos ; clrf Pos +1 ; clrf NewPos ; clrf NewPos + 1 ; movlw 31 ; This is the maximumposition for a half movwf MaxPos ; stepped 400 S/R motor i.e. :799 movlw 3 ; Adjust if you use another type motor movwf MaxPos + 1 movlw 144 ; This is the number of steps to go half movwf HalfWay ; way around for a 400S/R motor movlw 1 ; adjust if using another type motor movwf HalfWay + 1 movlw 3 ; this will force motor position reset movwf fSeekzero movlw 0x41 movwf BasePtr clrf DAToffset clrf Data0 clrf Data1 clrf Data2 clrf Data3 clrf Data4 clrf Data5 clrf Data6 clrf Data7 clrf fQuery ;**************************************************** ; This portion initializes the position ; in a frame data transfer and ; the PICID2 from values stored in EEPROM ;**************************************************** movlw 0x020


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movwf Name2BasePtr bcf STATUS, RP0 ; assure we are in block 0 clrf NB2offset INITID2 movf Name2BasePtr, w movwf FSR movf NB2offset, w addwf FSR, f ; after this FSR points to registers bsf STATUS, RP0 ; starting at 0x20 movwf EEADR ; and EEADDR to char in EEPROM bsf EECON1, RD ; command a read movf EEDATA, w ; pick up the character bcf STATUS, RP0 movwf INDF ; and do an indirect write to registers incf NB2offset, f movlw 15 subwf NB2offset, w ; see if we are done btfss STATUS, Z goto INITID2 ;*********** End of EEPROM reading ****************** movlw 'Z' ; Initialize the ID to ZZZ99 movwf NameByte0 movwf NameByte1 movwf NameByte2 movlw '9' movwf NameByte3 movwf NameByte4 movlw 0x032 ; Housekeeping variable set up movwf NameBasePtr clrf NBoffset movlw 0x029 movwf Name2BasePtr clrf NB2offset clrf fCMD clrf fDATA clrf DAToffset ; initalize PORTB<7:4> as outputs, PORTA<3:0> as inputs movlw 7 ; turn off the input comparators movwf CMCON ; or PA inputs won't respond properly bsf STATUS, RP0 ; select page 1 for TRIS access movlw 0x0F movwf TRISB ^ 0X080 ; movlw 0xFF movwf TRISA ^ 0X080 bcf STATUS, RP0 ; back to page 0


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; Turn on timer 2 bsf STATUS, RP0 movlw 190 movwf PR2 ^ 0x080 bcf STATUS, RP0 movlw 0x0F movwf T2CON ; prescale = 16, TMR2 on, postscale = 2 ; set up the USART bsf STATUS, RP0 movlw 77 ; for 2400 baud with 12 MHz clock movwf SPBRG ^ 0x080 movlw (1 << TXEN) ; set transmit enable movwf TXSTA ^ 0x080 bcf STATUS, RP0 movlw (1 << SPEN) | (1 << CREN) movwf RCSTA ; enable serial port and receive functionality ; turn on interrupts bsf INTCON, GIE ; enable global interrupts bsf INTCON, PEIE bsf STATUS, RP0 bsf PIE1 ^ 0x080, TMR2IE ; enable interrupts from TMR2 bcf STATUS, RP0 ;************************************************** ; This is the "idle loop" where the PIC waits for ; something interesting to happen ;************************************************** LOOP bsf INTCON, GIE ; turn interrupts back on IDLELOOP btfsc PIR1, RCIF goto SERIALIN btfsc fQuery, 7 goto QUERY goto IDLELOOP ;***************** End of idle loop *************** SERIALIN bcf INTCON, GIE ; turn off global interrupts movf RCREG, w ; pick up the received character movwf RXBUF ; and stick in the receive buffer


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bcf PIR1, RCIF ; clear the received character flag btfsc fDATA, 0 ; test if handling data yet goto HANDLEDATA btfsc fCMD, 7 ; test if this is a command byte goto HANDLECMD ; This portion checks incoming byte stream for ID match ; first check against name 1 movf NameBasePtr, w ; Generate the address into the ID movwf FSR ; and stuff it into the FSR movf NBoffset, w addwf FSR, f movf INDF, w ; Next ID byte is now in W subwf RXBUF, w btfss STATUS, Z ; zero if character matches goto NOMATCH1 incf NBoffset, f movlw 5 subwf NBoffset, w ; check if all characters of ID have btfss STATUS, Z ; scanned. If not, we're done for now goto CHECK2 clrf NBoffset ; if we got here, the character stream clrf NB2offset movlw 0x0FF ; in matched the unit ID. Next byte in movwf fCMD ; will be a command. goto LOOP NOMATCH1 clrf NBoffset ; Character doesn't match so no ID1 match CHECK2 movf Name2BasePtr, w ; generate the address into ID2 movwf FSR ; and stuff it into the FSR movf NB2offset, w addwf FSR, f movf INDF, w ; Next ID byte is now in W subwf RXBUF, w btfss STATUS, Z ; zero if character matches goto NOMATCH2 incf NB2offset, f movlw 5 subwf NB2offset, w ; check if all characters of ID have btfss STATUS, Z ; scanned. If not, we're done for now goto LOOP clrf NBoffset ; if we got here, the character stream clrf NB2offset movlw 0x0FF ; in matched the unit ID. Next byte in movwf fCMD ; will be a command. goto LOOP NOMATCH2


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clrf NB2offset goto LOOP ; start over HANDLECMD movf RXBUF, w movwf fCMD ; Store command movlw 0x0FF ; and set fDATA flag movwf fDATA movlw HIGH GETXFERCNT movwf PCLATH movf fCMD, w andlw 0x0f call GETXFERCNT ; Initialize data transfer count for the movwf DataCtr ; command movf DataCtr, w btfsc STATUS, Z goto CMDPREP clrf DAToffset clrf SkipCtr btfss DataCtr, 7 goto LOOP bcf DataCtr, 7 movf SkipCt, w movwf SkipCtr goto LOOP GETXFERCNT ; This table returns expected byte count movwf Temp movlw HIGH XFERCOUNT movwf PCLATH Movf Temp, w addwf PCL, f ; to receive for each command, plus flag XFERCOUNT retlw 0x00 ; Command 0 in bit7 indicating frame retlw 0x02 ; Command 1 transfer retlw 0x82 ; Command 2 *** this is a frame transfer*** retlw 0x00 ; Command 3 retlw 0x00 ; Command 4 retlw 0x00 ; Command 5 retlw 0x00 ; Command 6 retlw 0x00 ; Command 7 retlw 0x00 ; Command 8 retlw 0x05 ; Command 9 5 bytes for rename retlw 0x01 ; Command 10 1 byte for skip count value retlw 0x00 ; Command 11 retlw 0x00 ; Command 12 retlw 0x00 ; Command 13 retlw 0x00 ; Command 14 retlw 0x00 ; Command 15 HANDLEDATA ; This implements skipping data bytes


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movf SkipCtr, f ; for the frame data transfer btfsc STATUS, Z goto STUFFDATA decf SkipCtr, f goto LOOP STUFFDATA movf BasePtr, w ; get the pointer into the display data movwf FSR movf DAToffset, w addwf FSR, f movf RXBUF, w ; grab the new byte movwf INDF ; and stuff it in incf DAToffset, f decfsz DataCtr, f ; All bytes transferred? goto LOOP CMDPREP clrf DAToffset clrf fDATA movlw HIGH CMDBRANCH movwf PCLATH movf fCMD, w andlw 0x0f addlw LOW CMDBRANCH btfsc STATUS, C incf PCLATH, f movwf PCL CMDBRANCH goto INITIALIZE ; CMD 0 goto SETMOTORPOS ; CMD 1 goto SETMOTORPOS ; CMD 2 goto INITIALIZE ; CMD 3 goto INITIALIZE ; CMD 4 goto INITIALIZE ; CMD 5 goto INITIALIZE ; CMD 6 goto INITIALIZE ; CMD 7 goto QUERYNAME ; CMD 8 goto RENAME ; CMD 9 goto SETFRAMESLOT ; CMD 10 goto INITIALIZE ; CMD 11 goto INITIALIZE ; CMD 12 goto INITIALIZE ; CMD 13 goto INITIALIZE ; CMD 14 goto INITIALIZE ; CMD 15 ;****************************************************** ; SETMOTORPOS checks the new position to see if ; it exceeds the step count for a full rotation. If so ; it loads the max position value in the new position ; variables. If it does not exceed it loads the new values ;******************************************************


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SETMOTORPOS movf Data0, w ; check low byte subwf MaxPos, w btfss STATUS, C ; carry set means a positive result goto BORROW1IN ; carry is clear so must borrow movf Data1, w ; carry set so no need to borrow subwf MaxPos + 1, w ; check high byte btfss STATUS, C ; carry set means positive result goto INTOOBIG ; carry is clear so result is negative INPUTOKAY movf Data0, w ; input not too big so just load movwf NewPos ; new values are go back to idle loop movf Data1, w movwf NewPos + 1 goto LOOP BORROW1IN movf Data1, w ; Underflow without carry means too big subwf MaxPos + 1, w btfss STATUS, C ; carry set means positive result goto INTOOBIG movwf Temp ; no underflow means much check after borrow movlw 1 subwf Temp, w btfsc STATUS, C ; carry clear means negative result goto INPUTOKAY ; carry set means positive result INTOOBIG movf MaxPos, w movwf NewPos movf MaxPos+1, w movwf NewPos + 1 goto LOOP ;***************** End of SETDISPLAYVAL ****************** QUERYNAME movlw 8 movwf EEPtr ; Pointer into EEPROM movlw 17 movwf QueryCtr ; Number of bytes to transfer movlw 0x81 movwf fQuery clrf fCMD goto LOOP SETFRAMESLOT movf Data0, w


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movwf SkipCt movlw 0x08 movwf EEPtr movlw 0x28 movwf RegPtr movlw 0x01 movwf Count clrf fCMD goto BURN RENAME movf Data0, w movwf Name2Byte0 movf Data1, w movwf Name2Byte1 movf Data2, w movwf Name2Byte2 movf Data3, w movwf Name2Byte3 movf Data4, w movwf Name2Byte4 movlw 0x09 movwf EEPtr movlw 0x29 movwf RegPtr movlw 0x05 movwf Count clrf fCMD ;******************************************************* ; This will copy a block of data in registers pointed ; to by "RegPtr", of size "Count", into EEPROM memory ; at a location pointed to by "EEPtr". ;******************************************************* BURN movf RegPtr, w movwf FSR movf INDF, w ; pick up a byte from registers bsf STATUS, RP0 ; EEPROM registers are in bank 1 movwf EEDATA bcf STATUS, RP0 movf EEPtr, w bsf STATUS, RP0 movwf EEADR bcf EECON1, WRERR bsf EECON1, WREN ; enable write movlw 0x055 ; required magic sequence for write movwf EECON2 movlw 0x0AA movwf EECON2


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bsf EECON1, WR bcf EECON1, WREN ; clear write enable since its going bcf STATUS, RP0 btfss PIR1, EEIF ; poll for EEPROM write completion goto $ - 1 bcf PIR1, EEIF ; clear the done flag incf EEPtr, f incf RegPtr, f decfsz Count, f goto BURN clrf NB2offset clrf fCMD clrf fDATA goto LOOP ;****************** End of write to EEPROM *************** ;********************************************************* ; QUERY services requests from the host. ; fQuery, 7 indicates an outstanding query. ;********************************************************* QUERY bcf INTCON, GIE ; turn off global interrupts btfss PIR1, TXIF ; Is serial transmit buffer ready? goto LOOP ; No, back to LOOP movf EEPtr, w bsf STATUS, RP0 ; movwf EEADR ; and EEADDR to char in EEPROM bsf EECON1, RD ; command a read movf EEDATA, w ; pick up the character bcf STATUS, RP0 movwf TXREG incf EEPtr, f decfsz QueryCtr, f goto LOOP clrf fQuery goto LOOP ;******************* End of QUERY ************************ org 0x02100 ; This is the start of the EEPROM area de 0 de 0 de 0 de 0 de 0 de 0 de 0 de 0 de 0 ; position in frame data transfer de "DG001 STPR V1.000" end


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Quantity Item Sources, notes, etc. .050”, .020” sheet aluminum Aircraft Spruce and Specialty, Enco .125” clear acrylic sheet .25” clear acrylic sheet

Local hardware store, Tap Plastics

4 5/8” #4-40 flat head screws 4 ¼” #4-40 round head screws

Local hardware store, Digikey, Mouser

1 Knob of 1/8” shaft 4 1.5” long, .25” hex spacers,

female-female 4-40 thread 8 1.00” long, .25” hex spacers,

male-female 4-40 thread

Jameco, Digikey, Mouser

1/8” brass hobby rod 5/32” brass hobby tubing

Hobby shop, Small Parts

Quick setting, filled epoxy Any number of brands will work as long as it has a filler. This makes the epoxy less likely to run and strengthens the cured material.

White paint, enamel Flat black paint, enamel

Hobby shop

1 400 step/rev bipolar stepping motor

1 7805 5 volt regulator This is for the motor. See text. 1 78L05 5 volt regulator 1 PIC16F628-20 micro controller 1 MAX232CPE RS-232C xcvr 1 L293D H-bridge driver 1 12.0 MHz ceramic resonator

with internal capacitors (Digikey X907-ND or eqv.)

1 H21A1 optical interrupter 4 1N4001 diode 1 Rotary encoder, Bourns 3315Y-

1-006 or eqv. 4 High efficiency red LED 1 330 ohm, ¼ watt resistor 1 390 ohm, ¼ watt resistor 4 10K ohm, ¼ watt resistor 2 100 ufd, 35 volt capacitor 1 10 ufd, 35 volt capacitor 4 .1 ufd, 50 volt capacitor 4 1 ufd, 50 volt capacitor 1 .01 ufd, 50 volt capacitor 1 DB9 male panel mounting

connector with solder lugs, mounting hardware

Perf board, wire, solder, etc.

Jameco, Digikey, Mouser, etc.

Sample Chapter�

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,�� You can order copies of this book by filling out the following form and mailing it along with payment. For on line ordering information check www.mikesflightdeckbooks.com.

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